This invention relates to a high speed lithographic (litho) film element comprising a film support containing one or more light-sensitive silver halide emulsions coated thereon.
The sensitometric curve which is characteristic of light-sensitive silver halide emulsions used in lithographic film elements conventionally has a sharp toe and high gradient or contrast. These elements also have a high top density and a very low fog. In order to achieve these results high chloride emulsions are conventionally employed.
These film elements are processed, after image exposure, in so-called "lith developers" which are based mainly on hydroquinone, have a low sulfite content and usually contain formaldehyde. These developers require a relatively long induction period prior to development, whereupon so-called "infectious development" occurs, giving rise to the specific sensitometric properties described above. These properties are necessary in order to achieve good dot quality, e.g., dots of high density and good edge sharpness as required for use in lithography.
Long induction periods are a serious drawback in this film/developer system. A large number of compounds have been reported in the prior art as accelerators for litho development and are said to reduce the induction period and thus increase the effective speed of the system. For example, onium compounds such as ammonium-, phosphonium- and sulfonium- compounds, pyridinium salts, and organic thiocompounds are disclosed as accelerators when added to the emulsions as reported in U.S. Pat. No. 3,785,822. These compounds are, however, not completely satisfactory. For example, the onium compounds either exhibit only a slight effect on the induction period or are not compatible with anionic wetting agents frequently used in these emulsions and form undesirable deposits with these wetting agents. Although the organic thiocompounds generally are useful in overcoming the induction period and increasing the system speed, they usually impair the dot quality and thus produce an unacceptable final image. Additionally, there is a tendency for the accelerators of the prior art to diffuse from the emulsion layer and to accumulate in the processing baths and deteriorate dot quality. As the accelerators accumulate, solids are formed by precipitation from the processing fluids. These solids also cause problems on the film surface and interfere with the dot quality in addition to causing sludge which affects processing conditions, especially in automatic machine processors. Thus, there is a pressing need for accelerators in the litho system which do not produce the serious side effects mentioned above.